The Chevy Tahoe Hybrid delivers a dramatic improvement in fuel economy over non-hybrid models – a 50-percent improved mileage in city driving – with all of the comfort and capability customers expect in a full-size SUV.

Indeed, the Tahoe Hybrid is mostly unchanged from the gasoline-only models in the key capability areas. There is no difference in passenger seating capacity, and towing and payload are highly competitive with other non-hybrid SUVs in the segment. There are some key subtle enhancements, however, in specific areas that contribute to the fuel economy improvements associated with the Tahoe Hybrid.

“Customers who demand better fuel economy, but need the capability of a full-size SUV will find exactly what they’re looking for in the Tahoe Hybrid,” said Ed Peper, Chevrolet general manager. “It offers the same great spaciousness as the non-hybrid models, with comparable cargo space and towing capability.”

The Tahoe Hybrid’s vastly improved fuel economy over gasoline-only siblings is due largely to a s tate-of-the-art hybrid system using GM’s all-new Electrically Variable Transmission (EVT). Two-wheel-drive Tahoe Hybrid models, for example, deliver 21 mpg in the city, a 50-percent improvement over their 5.3L non-hybrid siblings and comparable to the city fuel economy of much-smaller cars.

An industry first for any full-size SUV, the patented hybrid system allows these vehicles to return exceptional fuel economy in both city and highway driving while still delivering full-size SUV performance. Imagine a full-size SUV that offers 25- to 30-percent more combined city-highway fuel economy while also providing up to 6,200 pounds (2,812 kg) of usable towing capacity (2WD) and 6,000 pounds (2,722 kg) on 4WD models; 12,000 pounds (5,443 kg) GCWR and eight-passenger seating capacity.

Making all this possible is GM’s advanced EVT and 300-volt nickel-metal hydride Energy Storage System (ESS), which work in concert with the standard 6.0L V-8 gasoline engine with Active Fuel Management (AFM) and late intake valve closing (LIVC) technology. GM’s new hybrid system not only enables the Tahoe to launch and drive up to 30 mph on electricity alone, it also allows the 6.0L V-8 engine to operate in its more economical V-4 mode for longer periods.

EPA mileage ratings

Along with Active Fuel Management, the Tahoe Hybrid uses specific aerodynamic aids and reduced-mass components to help achieve EPA ratings of 21 mpg in the city and 22 on the highway for 2WD models. With 4WD models, the fuel savings are equally impressive, with the Tahoe Hybrid rated at 20 mpg in both city and highway driving.

Driving for best economy

By using the analog economy gauge found in the instrument cluster, drivers can maintain the most efficient driving style by keeping the gauge at its optimum economy indication as often as possible. Also, any time the gasoline engine is shut down — indicated when the tachometer needle points to the Auto Stop position, just below the zero mark — the driver will know when the vehicle is being powered only by electricity, which is the most efficient propulsion method.

Powertrain details

Drawing on its vast experience in developing hybrid bus propulsion systems, GM designed the EVT to provide the best combination of city and highway fuel economy. Used in concert with the Vortec 6.0L V-8 engine with Active Fuel Management, this all-purpose system is well-suited to provide excellent economy and full-size SUV passenger-carrying, cargo-hauling and trailer-towing capabilities.

Furthermore, the EVT is designed to bolt directly to the standard four-wheel-drive transfer case found on the gasoline-only models for true four-wheel-drive capability. The following major components comprise the Tahoe Hybrid’s unique powertrain system:

Electrically variable transmission (EVT): The key to GM’s all-new EVT is its unique assemblage of two 60 kW electric motors, three planetary gearsets and four traditional hydraulic wet clutches. This arrangement allows continuously variable operation, as well as providing four fixed gear ratios (with operation comparable to that of a standard electronically controlled automatic transmission).

A sophisticated Hybrid Optimizing System constantly receives torque-based data from the powertrain and other vehicle systems, and then determines the most efficient means of propelling the vehicle — either via electric power, gasoline engine power or a combination of the two. The EVT is like having two transmissions in one — continuously variable drive for light-load conditions and fixed-ratio drive for high-load situations.

In Mode One (M1), the EVT provides infinitely variable drive up to 1.70:1, while Mode Two (M2) provides ratios from 1.70:1 to 0.50:1. These two modes include all instances of electric-only (Auto Stop) and electric-gasoline hybrid operation. Depending on driving conditions and vehicle load, a fixed first-gear (G1) ratio of 3.69:1 is available (such as for pulling a large trailer) instead of the variable M1 ratios.

Similarly, a second-gear (G2) fixed ratio of 1.70:1 may be selected by the HOS, if needed, before transitioning to the variable M2 ratios. As road speed increases, and depending on driving and vehicle load conditions, variable-ratio M2 operation can be switched instantly to a third-gear (G3) fixed ratio of 1.00:1, and finally to a fourth-gear (G4) fixed ratio of 0.73:1, which is used mainly for steady-state highway cruising.

Unlike less advanced hybrid systems, the GM system occasionally shuts down the electric motors, allowing the EVT to function as a conventional automatic transmission. Typically, this fixed-ratio operation occurs at highway speeds or when hauling heavy loads, and can actually be more efficient than operating in electric-gasoline hybrid mode under the same conditions. This is because under high-load situations, when the fixed gears are in use, the electric motors can be used to generate electricity — or, if needed, they can be called on to supply additional torque for improved performance.

Energy storage system (ESS): Providing power to the EVT’s two electric motors is a 300-volt nickel-metal hydride Energy Storage System (ESS). This battery pack is located under the second-row seat, where it takes up virtually no additional space and does not interfere with second- or third-row ingress/egress — the fold-and-tumble capability of the second-row seat is maintained, and when the seat is flipped forward, a flat load floor is provided from the liftgate forward to the front seats.

The primary function of the ESS is to provide power (300 volts) to the EVT via the Traction Power Inverter Module (TPIM) and to store captured energy produced during regenerative braking. The ESS can also be charged, when necessary, by the gasoline engine via one of the two electric motors when operated in generator mode.

In addition to supplying power to the EVT, the ESS also provides power to the air conditioning compressor and the Accessory Power Module (APM), which converts the high-voltage supply to 42 volts for the electric power steering system, and 12 volts for the vehicle battery and other 12-volt electrical accessories.

Battery pack durability and reliability are maintained via optimized charge and discharge cycles, as well as a dedicated cooling system that draws air from the passenger compartment. As part of the vehicle’s emission control system, the ESS is warranted for eight years/100,000 miles.

Vortec 6.0L V-8 engine: The Vortec 6.0L V-8 gasoline engine is used in the Tahoe Hybrid because of its favorable torque characteristics. This is especially important because GM engineers adapted the 6.0L V-8 to operate with late intake valve closing (Atkinson-cycle combustion process) for reduced pumping losses and better overall fuel economy.

In addition, the Vortec 6.0L engine features variable valve timing to control late intake closing, as well as Active Fuel Management, which allows four of the eight cylinders to be shut off during periods of light load. With the benefit of electric boost, the engine is able to stay in V-4 mode for longer periods in the Tahoe Hybrid.

Auto Stop mode

A key contributor to fuel economy is the gasoline engine’s Auto Stop mode. Once the vehicle reaches 0 mph, the gasoline engine is automatically shut down. By leaving the engine off and allowing the vehicle to move only under electric power up to 30 mph, such as during heavy stop-and-go traffic, fuel consumption is greatly reduced.

Typically, the hybrid control system will shut off the gasoline engine when it is not needed for propulsion. However, there may be instances when the engine will remain running after it is started:

The engine or transmission is not fully warmed up

The transmission or ESS temperature is high

The ESS charge level is low and requires recharging

The hood is not fully closed

The “Service Engine Soon” light is illuminated

Situations in which the engine will restart include:

The accelerator pedal is depressed in demand for heavy acceleration

The engine must run to meet the current climate control settings

The ESS charge level is low and requires recharging

Regenerative braking

The regenerative braking system works in concert with the hydraulic braking system. The system uses a brake pedal emulator, which provides progressive resistance as the pedal is depressed. This consistent pedal feel is very much like that of a standard hydraulic braking system in operation.

Because there are two braking systems — regenerative and hydraulic — more braking power is available than that found on gasoline-only models. Because of this, braking distances will be noticeably shorter.

From time to time you may hear the computer-controlled pump re-pressurizing the hydraulic brake accumulator. This is normal and necessary to the safe operation of the vehicle. Note that actuating the brake pedal repeatedly will deplete the accumulator pressure more quickly and cause the pump to cycle more often.

Peace and quiet

The superior passenger comfort of both these SUVs has been maintained throughout, with a quiet cabin environment at the very top of the list. Tahoe Hybrid models match their gasoline-only siblings in comfort and convenience, but attention to acoustic details makes these SUVs even more enjoyable.

Some of the significant accomplishments in noise reduction include the following:

New exhaust system and resonator — Specially tuned for the 6.0L LIVC engine’s Active Fuel Management operation, this new exhaust system ensures comfortable interior acoustics and a pleasant exhaust note during both V-4 and V-8 operation

Electrically driven 300-volt air conditioning compressor reduces vibration and allows the HVAC system to cool the passenger compartment even when the gasoline engine is shut off. Includes the standard tri-zone HVAC system

Electrically driven 42-volt variable-assist power steering reduces vibration and provides up to a 0.5-mpg fuel economy improvement by reducing parasitic losses common in belt-driven hydraulic systems

Energy Storage System (ESS) cooling system internal fan is tuned to be quiet at low vehicle speeds when the fan could more easily be heard by the occupants

Hybrid interior features

A unique hybrid gauge cluster includes a special tachometer incorporating an Auto Stop position just above the 0 rpm mark that indicates when the gasoline engine is in shut-down mode. Also part of the cluster is an analog economy gauge located to the left of the tachometer that helps the driver maintain the most efficient driving style.

Located midway down the center stack, the standard navigation system’s 6.5-inch full-color VGA display doubles as a monitor that shows a graphic representation of the hybrid drivetrain’s power flow. This display continuously informs the occupants how the system is sending power to the wheels from the ESS/electric motors, the gasoline engine or both. The screen also displays reverse electrical flow during regenerative braking and whether the vehicle is in two- or four-wheel-drive mode.

Aerodynamic exterior

While the Tahoe Hybrid bears a close resemblance to other models, styling revisions are made for aesthetic and functional purposes. In addition to setting these models apart from other SUVs, the following exterior styling changes contribute to improved fuel economy:

The front fascia is devoid of fog lamp and tow hook openings, while a lower air dam helps redirect air flow from underneath the vehicle to reduce turbulence and associated drag.

Subtle, yet important aero tuning details around the wheel openings and other areas all contribute to a low 0.34 drag coefficient

A slightly larger front grille opening allows increased airflow to the radiator and engine compartment, helping to cool the advanced hybrid power electronics

The roof rack side rails have been eliminated to reduce drag and provide a clean, uninterrupted appearance

Mass is an important factor in overall vehicle efficiency. Because of the slight increase in curb weight resulting from the various hybrid components, engineers took great efforts to reduce the mass of numerous vehicle components, keeping the Tahoe Hybrid in the same test weight class as its gasoline-only counterparts.

On the inside, special thin-profile, lighter-weight front seats provide all the function and comfort of those found in the gasoline-only Tahoe. Another advantage of these new seats is the additional 1.5 inches (38 mm) of second-row passenger knee room.

Additional mass was eliminated by adapting a tire inflator kit that takes the place of the spare wheel and tire, hoist mechanism, jack and tools found on gasoline-only models. Easy to use, the tire inflator kit is conveniently located behind the left-rear quarter trim panel in the cargo area. And, the inclusion of standard OnStar with Stolen Vehicle Slow Down, with one-year subscription to Directions & Connections with Turn-by-Turn Navigation, adds peace of mind should the driver require roadside assistance.

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